Cardiac arrhythmias are a significant health problem, and atrial fibrillation is a common cardiac arrhythmia. Atrial arrhythmias may increase risk factors for various conditions such as embolisms and can contribute to the onset of ventricular arrhythmia.
It is believed that cardiac electrical impulses start in a sinoatrial (SA) node, spread through the atria, and progress through the atrial-ventricular (AV) node to the ventricles to complete a heartbeat. Atrial fibrillation is an irregular heart rhythm that originates in the atria or the upper two chambers of the heart. The pulmonary veins, in particular, can be sources of disruptive re-entrant electrical impulses.
One known manner of treating atrial fibrillation is by use of medication that is intended to maintain a normal sinus rate and/or decrease ventricular response rates. It is also known to use implant devices such as atrial pacemakers for this purpose. Further, other known methods and devices have been developed for creating therapeutic lesions, e.g., by minimally-invasive surgical methods, in the myocardial tissue to block unwanted electrical impulses that are believed to be the source of atrial fibrillation. In this context, ablation has come to mean the deactivation, or removal of function, rather than the actual removal of tissue. A number of energy sources may be used for creating these “blocking” lesions that are preferably transmural and extend across the entire heart wall.
Formation of lesions may be performed using both endocardial and epicardial devices and techniques. Endocardial procedures are performed from within the heart. Since the endocardium primarily controls myocardial functions, there are inherent advantages to generating lesions by applying an energy source to endocardial surfaces. One known manner of applying energy for this purpose is utilizing radio frequency (RF) catheters. Other known endocardial ablation devices include expandable balloons that are inflated with a cryogenic fluid or coolant, such as nitrous oxide. Examples of known lesion formation devices, including cryogenic balloon devices for use in endocardial ablation and their operation are described in U.S. Patent Application Publication No. 20060084962, U.S. Pat. Nos. 6,027,499; 6,468,297; 7,025,762; 7,081,112; 7,101,368 and 7,150,745, the contents of which are incorporated herein by reference.
For example, referring to FIG. 1, a system 100 for cryogenically ablating tissue utilizing a cryogenic balloon catheter 110 includes a source 120 of coolant or refrigerant 122 such as nitrous oxide or another suitable flowable coolant (generally referred to as coolant 122). During use, a cryogenic balloon catheter 110 is positioned within a desired location within a patient utilizing a guide wire 112 that extends through a guide wire tube, lumen or conduit 113. Coolant 122 is delivered through a console or an interface 130 and one or more connectors or tubes 140 to the balloon catheter 110 to inflate the expandable or balloon element 114 (generally referred to as balloon element 114) and cryogenically ablate adjacent tissue surrounding the chilled balloon element 114 or a portion thereof. Cryogenic cooling results from a pressure drop as the coolant 122 is sprayed into an inner space 116 defined by the balloon element 114, thereby causing the balloon element 114 to expand against and chill adjacent target tissue. During the procedure, the vacuum level within the balloon element 114 may be controlled using a vacuum source 150, and spent coolant 122 is evacuated from the balloon catheter 110 through the exhaust 160 or another suitable tube that may be a non-coaxial tube.
The effectiveness of balloon catheters 110 depends on various factors including, for example, the manner in which coolant 122 is distributed within the balloon element 114. More specifically, the effectiveness of chilling the balloon element 114 and cryo-ablation of adjacent tissue may depend on how uniform the temperature is along an inner surface 118 of the balloon element 114. Non-uniform temperatures may be caused by exposure to non-uniform or inconsistent or uneven coolant 122 flows, thereby resulting in temperature variations along the inner surface 118 and non-uniform chilling and cryo-ablation of tissue.
For example, referring to FIG. 2, a coil-shaped hypotube or coil 200 (a portion of which is illustrated) may be used to deliver and dispense coolant 122 to inflate the balloon element 114. FIG. 2 illustrates a portion of a coil-shaped hypotube 200 shaped to have an inner coil 210 and an outer coil 220 through which one or more straight apertures or holes 230 are drilled. Other straight apertures 230 (not shown in FIG. 2) may also be drilled through the outer coil 220. With this configuration, a radial line R extends from a central axis CA defined by the hypotube coil 200 and through the straight hole 210 such that the coolant 122 is dispersed perpendicularly 232 through the straight aperture 230 relative to an outer surface 222 of the outer coil 220 and into a space 242 defined between the outer coil 220 and the balloon element 114, thereby inflating the balloon element 114 and chilling and cryogenically ablating adjacent tissue.
As another example, referring to FIG. 3, other cryo-ablation devices may utilize a non-coiled tube 300 that also includes straight apertures 330a-d formed through the tube 300 such that coolant 122 is dispersed through the straight apertures 330 perpendicularly relative to the outer surface of the tube 300. The same flow characteristics described below may apply to both types of tubes 200, 300.
With the straight holes or apertures 230, 330 of known coolant delivery tubes 200, 300, coolant 122 is dispersed against the inner surface 118 of the balloon element 114 in an uneven manner. For example, with reference to FIGS. 3 and 4, as coolant 122 is dispersed through the straight apertures 330, the coolant 122 is initially concentrated at cold spots 410 to cool a section of the inner surface 118 of the balloon element 114, which also cools adjacent surfaces 412. However, initially, the temperature along the balloon inner surface 118 varies, e.g., sections 414 are warmer than the concentrated cold spots 410 and other cooled sections 412. For example, the temperature of a concentrated cold spot 410 may be about −80° C. whereas a temperature of a cooled section 412 may be about −20° C., and a temperature of a warmer section 414 may be about 0° C. to 37° C. Over time, this temperature differential may be reduced with continued cooling, but smaller temperature variations may still exist across the inner surface 118 of the balloon element 114, as shown in FIG. 5, thereby resulting non-uniform and non-uniform chilling and ablation of tissue. Further, while it may be possible to reduce these non-uniform cooling effects over time, doing may require longer procedures and larger quantities of coolant 122.
Further, known devices may require large amounts of coolant 122, a large number of nozzles and longer treatment times to compensate for uneven coolant distribution and cooling as shown in FIGS. 4-5. For example, large amounts of coolant 122 may be utilized to “overtreat” a tissue region, e.g., to form a cold spot 410 with the hope that passive conduction in the tissue will eventually migrate and fill in the space between a first tissue region or cold spot 410 and second tissue region or cold spot 410. For example, it has been demonstrated in experiments on animals that such passive conduction techniques may adequately fill a gap of about 6 mm between nozzles or between cold spots. However, certain known balloon elements 114 are about 23 mm in diameter and have a circumference of about 70 mm. Using nozzles that are spaced apart by about 6 mm (to achieve sufficient passive conduction), however, would require a minimum of 11-12 nozzles. Larger balloon elements 114, e.g., having a diameter of about 28 mm and an even larger circumference, would require even more nozzles. In these cases, larger quantities of coolant 122 are required, and such techniques require longer treatments to cool adjacent tissue regions.
Further, overpowering a balloon element 114 with additional coolant 122 may result in puddling or accumulation of liquid coolant 122 in the bottom of the balloon element 114. This accumulation of coolant 122 contributes to uneven treatment and may also pose safety risk if the catheter exhaust lumen becomes plugged or the balloon element 114 ruptures since 1 cc of liquid coolant may evaporate into about 700 cc of gas.